1. Field of the Invention
The present invention relates in general to a magnetic recording medium and, in particular, to a high performance and highly reliable magnetic recording medium that exhibits low noise and sufficiently high coercive force and is mounted on a variety of magnetic storage devices, such as an external storage device of a computer.
2. Description of the Related Art
A magnetic storage device is one of information recording devices that support a highly informational society in recent years. With an increase of information to vast amounts, magnetic recording media used in magnetic storage devices are forced towards a higher recording density and lower noises. Achieving the high recording density needs minimization of a unit size of inversion of magnetization, which in turn requires a minute magnetic grain size. Lowering of noises needs a reduction of a fluctuation of magnetization due to a magnetic interaction between grains in addition to the minute magnetic grain size.
A magnetic layer using a CoCrPt alloy in a conventional medium contains a great amount of chromium to segregate enough chromium towards a grain boundary region to achieve noise reduction. As a result, a magnetic anisotropy constant Ku of the magnetic grains is lowered, and a thermal stability of the medium deteriorates. On the other hand, if a platinum content is increased aiming at enhancement of the Ku value, the chromium segregation to the grain boundary region is hindered, resulting in an increase of media noise. Thus, there exists a trade-off relationship between the thermal stability and the noise reduction.
To address the problems set forth, a variety of proposals have been made including compositions and structures of the magnetic layer, and materials for a nonmagnetic underlayer and a seed layer. Specifically, a type of medium has been proposed having a so-called granular magnetic layer including magnetic crystal grains and a nonmagnetic matrix of an oxide or a nitride surrounding the crystal grains. In the medium having the granular magnetic layer, the magnetic crystal grains are nearly perfectly isolated magnetically from each other by virtue of the intervening nonmagnetic substance. Because a minimum magnetization unit can be a size of an individual grain, which is 4 to 10 nm, for which a high recording density is possible. Furthermore, an exchange in an interaction between the crystal grains is expected to be suppressed by the nonmagnetic matrix surrounding each grain.
The granular magnetic layer inherently allows to avoid an addition of quantities of chromium because the grain boundary segregation is principally performed by silicon oxide contrary to a conventional CoCrPt magnetic layer. Because the silicon oxide is compelled to precipitate at the grain boundary in the granular magnetic layer, the grain boundary segregation is promoted without lowering the magnetic anisotropy constant Ku, which means compatibility with noise reduction. For higher recording density in the future, it is indispensable to make the magnetic layer thinner. The thinner magnetic layer has lower magnetic anisotropy energy KuV, where Ku: magnetic anisotropy constant and V: activation volume. To prevent lowering of the magnetic anisotropy energy, the magnetic anisotropy constant Ku must be increased, which requires an addition of an amount of platinum to the composition of the magnetic layer. Addition of large amounts of platinum inhibits the grain boundary segregation of chromium in the conventional CoCrPt magnetic layer and increases noises, in the granular magnetic layer. In contrast, because silicon oxide easily segregates to the grain boundary region even if plenty of platinum is added, the granular magnetic layer has an advantage that larger amount of platinum can be contained while maintaining isolation of magnetic grains.
U.S. Pat. No. 5,679,473, for example, discloses that low noise is achieved by a granular recording film having a structure in which each magnetic crystal grain is separated by nonmagnetic oxide surrounding the magnetic crystal grain. The granular recording film may be formed by deposition employing an RF sputtering using a target of CoNiPt with an additive of an oxide such as SiO2. In such a granular magnetic film, because a nonmagnetic and nonmetallic phase physically separates each magnetic grain, magnetic interaction between the magnetic grains diminishes. So, a formation of a zigzag magnetic domain wall is suppressed, which arises at a transition region of a recording bit, leading to achieving low noise characteristics.
Japanese Unexamined Patent Application Publication Nos. 11-213371 and 2000-123445 disclose depositing an underlayer with a body centered cubic structure prior to depositing the magnetic layer that has a hexagonal closed packed structure. Japanese Unexamined Patent Application Publication No. 2000-82210 discloses a magnetic recording medium provided with a nonmagnetic intermediate layer with a body centered cubic structure between the magnetic layer and the underlayer.